1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a light source device, a backlight assembly and a liquid crystal display device for preventing a light guide plate from being discolored due to light emitted by a lamp.
2. Description of the Related Art
Presently, information processing devices are developed for various shapes and functions and higher processing speed. The information processing devices convert various type of information into electrical signals. Display devices function as an interface between users and the information processing devices such that the users can identify the information processed by the information processing devices.
Recently, the display device manufacturing industry has been focusing on developing smaller and lighter liquid display devices. The liquid display devices provide richer and fuller colors and higher resolutions as compared with conventional display devices, such as a CRT (Cathode Ray Tube). As the result, the liquid display devices have been gaining its popularity among other display devices, for example, computer monitors, household television sets that are hung on a wall, and the like.
Generally, to operate a liquid crystal display device, a voltage is applied to the liquid crystal in the predetermined molecule arrays to change the liquid crystal in the other arrays, which alters the optical characteristics of the liquid crystal, such as a double refractivity, a rotatory polarization, a dichroism and a light scattering according to the molecule arrays of the liquid crystal to emit the light. Therefore, the liquid crystal display device can display the images according to the changes of the optical characteristics of the liquid crystal cell.
FIG. 1 is an exploded perspective view showing a liquid crystal display device according to the conventional art schematically, and FIG. 2 is a sectional view of showing the constructions of a lamp shown in FIG. 1. FIG. 3 is a graph for showing a discoloration of a light guide plate due to light supplied by the lamp shown in FIG. 1.
Referring to FIGS. 1 and 2, the liquid crystal display device comprises a liquid crystal display module for displaying images when image signals are applied thereto and a case (not shown) for accommodating the liquid crystal display module. The liquid crystal display module includes a display unit having a liquid crystal display panel for displaying the images.
The display unit 200 includes the liquid crystal display panel 210, a printed circuit board 220 for transferring data signals, a printed circuit board 212 for transferring gate signals, a tape carrier package 230 for transferring data signals and a tape carrier package 250 for transferring gate signals.
The liquid crystal display panel 210 includes a thin film transistor board 212 which is normally a transparent glass on which the thin film transistors are formed in a matrix, a color filter board 214 having RGB pixels that are formed thereon by a thin film process and present predetermined colors while the light passes through the color filter board 214, and liquid crystal (not shown).
When the thin film transistors of the thin film transistor board 212 are turned on, electric field is created between the pixel electrodes of the thin film transistor board 212 and the common electrodes of the color filter board 214. The electric field changes the liquid crystal layer's array angle, and changes the light transmittivity. As a result, it is possible to gain the desired pixels.
A driving signal and a timing signal are applied to the gate lines and data lines of the thin film transistor in order to control the array angle of the liquid crystal and the time of arraying the liquid crystal in the liquid crystal display panel 210. That is, the printed circuit boards 220 and 240 generate and apply the gate driving signal and the data signal for driving the liquid crystal display device and a plurality of timing signals for applying the gate driving signal and the data signal, to the gate lines and the data lines of the liquid crystal display panel 210.
The backlight assembly 300 is provided under the display unit 200 to supply the light to the display unit 200 uniformly. The backlight assembly 300 includes a lamp 310 for generating the light. The lamp 310 is protected by a lamp cover 312.
As shown in FIG. 2, the lamp 310 has a glass tube 301 in which inert gas is filled up and a hot electrode and a cold electrode 303 and 305 to which high voltage electricity and low voltage electricity are respectively applied are disposed at both ends respectively. Metal electrodes 303a and 305a are mounted at the hot and cold electrodes 303 and 305, respectively, in the glass tube. As outer voltages discharge electricity between the two electrodes, the inert gas 309 is excited and ultraviolet rays are generated from the inert gas 309. Some ultraviolet rays 311 are conversed into visible rays 313 by the fluorescent material in the glass tube 301 and then supplied to the light guide plate 320.
The light guide plate 320 has a size corresponding to that of the liquid crystal panel 210 of the display unit 200, which is disposed under the liquid crystal panel 210 to guide the light emitted by the lamp 310 toward the display unit 200 by changing a pathway of the light.
A plurality of optical sheets 330 are provided on the light guide plate 320 to make brightness of the light from the light guide plate 320 to the liquid crystal display panel 210 uniformly. In addition, a light reflecting plate 340 provided under the light guide plate 320 reflect leaking light to the light guide plate 320 so as to improve the efficiency of the light.
The display unit 200 and the backlight assembly 300 are supported by means of a mold frame 400 used as a receptacle. The mold frame 400 is provided with a top chassis 500 for preventing the display unit 200 from departing from the mold frame 400 while the printed circuit boards 220 and 240 are bent toward outside of the mold frame 400 and fixed to the bottom surface of the mold frame 400.
The light guide plate 320 of polymethyl methacrylate (hereinafter, referred to as PMMA) effects the size and the weight of the liquid crystal display unit. As the size of the light guide plate 320 is directly related to a size of the liquid crystal display panel, however, researches have been continued to produce a liquid crystal display device that is light, thin and small by reducing the weight of the light guide plate 320 as much as possible.
Recently, for an example, a light guide plate which uses cycloolefin polymer (hereinafter, referred to as COP) has been developed to reduce the weight thereof. As shown in FIG. 3, however, the COP light guide plate is susceptible to the ultraviolet rays in the light emitted by the lamp 310 as compared with PMMA light guide plate.
For an example, when the PMMA light guide plate was aged at an ordinary temperature for 3300 hours, the PMMA light guide plate had changes ΔX and ΔY in the order of 0.015 on X and Y color coordinates. However, the COP light guide plate had changes ΔX and ΔY in the order of 0.025 and 0.032 on X and Y color coordinates. This is caused by a recombination reaction of the polyolefin resin composition to the ultraviolet rays, and the light guide plate is subjected to discoloration into yellow when used for a long period of time.
Besides, as shown in a graph in FIG. 4, when aged for about 3000 hours, a maintenance rate of brightness for the PMMA light guide plate is about 70%, while a maintenance rate of brightness for the COP light guide plate is about 60%.
Hence, COP light guide plate has the problems of discoloration and degradation of brightness characteristics as aged.